In curtain coating, a travelling web or object is coated by a free-falling curtain of coating liquid that is caused to impinge onto the travelling web or object to form a layer thereon. The width of the free-falling curtain can be maintained by edge guides that are in adherent, i.e., wetting, contact with the lateral boundaries or edges of the curtain. For example, in aqueous solution systems for photographic products it is known to use a curtain coating method which comprises the simultaneous application of silver halide emulsions containing gelatin as a binder by using a slide hopper coating apparatus to form a multilayer photographic film or paper. A moving web is coated by a free-falling curtain of coating composition wherein the multilayer composition is formed on the slide hopper and caused to fall as a curtain and impinge the moving web to form the coated layer thereon. The formation of a composite of a plurality of distinct layers on a moving web is described in U.S. Pat. No. 3,508,947, which relates particularly to the manufacture of multilayer photographic materials such as photographic film and paper.
U.S. Pat. No. 3,508,947 describes a method and apparatus commonly used to start-up a continuous curtain coating process as shown in FIG. 1 herein. As shown in FIG. 1 herein, a flat curtain deflector 16, 22 is mounted in position 21 or 23 so it can be pivoted or slid into position. During preparation for coating, the deflector 16, 22 diverts the free fall of the curtain 12 fluids to flow down the deflector surface and into a catch pan 24. The coating process is started after establishing the stability of the curtain 12 and bringing the web 18 to a desired coating speed. The curtain deflector 16, 22 is then retracted by pivoting or sliding it away from the falling curtain 12 to allow the curtain 12 liquids to impinge onto the moving web 18. Although the curtain deflector 16, 22 can be located on either the front or back side of the falling curtain 12 as shown in positions 21 and 23, respectively, use of a deflector 16, 22 in this manner results in the deposition of excess coating liquids on the moving web 18.
Deposits of excess coating liquids on the moving web 18 often occur in amounts that result in the generation of large quantities of extremely costly waste, e.g., for each occurence it is not unusual to have more than a thousand linear meters of wasted coated photographic material. In addition, areas of excess coating liquids deposited on the web will often transfer from the web onto coating roller 20, the machine rollers used in transporting the moving web 18, and also the web wind-up end of the machine. Furthermore, it should be appreciated that each new start-up of a coating process is susceptible to incurring the same problems.
U.S. Pat. No. 4,851,268 describes a curtain coating start-up method and apparatus comprising a catch pan 30 as shown in FIG. 2 herein. The catch pan 30 includes a primary lip 32 and secondary lip 44 which, during retraction in the start-up phase of the curtain coating process, prevents the deposit of excess coating liquids onto the moving web and reduces any coated waste. Such catch pan configuration, as with other pan configurations, was found to not completely control the liquids adjacent to the edge guides during the catching of the curtain material before and during retraction of the catch pan. Therefore, efficient use of the curtain coating method for manufacturing, for example, photographic materials has thus far been adversely affected by the inability to develop efficient coating start-ups and shut-downs at the intended coating flow rate and web speeds, and control of the liquids adjacent to the edge guides, especially when the edge guides include the use of well known stripping or flushing liquids.